Marine Life: The Biological Diversity of Earth's Oceans

Marine life encompasses all organisms inhabiting salt water environments. This vast biological network includes aquatic animals, plants, algae, fungi, protists, single-celled microorganisms, and viruses. These organisms thrive in a variety of saline habitats, ranging from the open oceans and marginal seas to the brackish waters of estuaries, lagoons, and coastal wetlands.

As of 2023, scientists have documented more than 242,000 marine species, though estimates suggest up to two million species remain undiscovered. With an average of 2,300 new species described annually, the study of these organisms is a dynamic intersection of marine biology and biological oceanography.

By volume, the oceans provide approximately 90% of the living space on Earth. They have served as the cradle of life and vital sanctuaries throughout geological history.

Killer whales (orcas) are highly visible marine apex predators that hunt many large species. However, most marine activity takes place among microscopic organisms that cannot be seen individually with the naked eye, such as marine bacteria and phytoplankton.[1]

Key Facts

Elevation histogram showing the percentage of the Earth's surface above and below sea level

Living Space: Oceans account for about 90% of the habitable volume on Earth.
Biomass: Marine microorganisms are estimated to constitute between 70% and 90% of total marine biomass.
Species Count: Over 242,000 species are documented, with millions more potentially awaiting discovery.
Size Range: Organisms range from phytoplankton (0.02 micrometers) to the blue whale (up to 33 meters).
Climate Impact: Primary producers like cyanobacteria sequester carbon and produce oxygen via photosynthesis.

The Evolution of Ocean Life

Composition of seawater. Quantities in relation to 1 kg or 1 litre of sea water.

The earliest known life forms evolved as anaerobic prokaryotes (bacteria and archaea) around deep-sea hydrothermal vents during the Archean eon. These organisms lived without oxygen before the emergence of photoautotrophs, which allowed microbial mats to expand into shallow waters.

A pivotal moment occurred during the early Proterozoic with the Great Oxygenation Event. This shift in marine chemistry caused widespread extinction among anaerobes but facilitated the evolution of eukaryotes (organisms with complex cells) through symbiogenesis—a process where surviving anaerobes and aerobes merged.

Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.[58] The three domains are coloured, with bacteria blue, archaea green and eukaryotes red.

From the Avalon to the Cambrian Explosion

Complex life emerged from marine eukaryotes during the Neoproterozoic, leading to the Avalon Explosion, characterized by sessile (stationary) macrofauna. This was followed by the more prominent Cambrian Explosion in the early Phanerozoic, which saw the rise of actively moving eumetazoans.

These marine pioneers eventually expanded into fresh waters. During the Ordovician, fungi and green algae washed ashore, and by the Silurian and Devonian periods, they expanded inland, creating the foundation for terrestrial ecosystems.

Dickinsonia may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.

The Spectrum of Marine Organisms

Microorganisms and Cellular Life

The ocean is dominated by microscopic life. This includes viruses, such as cyanophages that infect cyanobacteria, and prokaryotes like the giant bacterium Thiomargarita namibiensis and methane-producing archaea.

These are cyanophages, viruses that infect cyanobacteria (scale bars indicate 100 nm)

Protists represent a diverse group of eukaryotic microorganisms. They are categorized by how they obtain nutrients:

Plant-like: Algae that use photosynthesis.
Animal-like: Protozoans that consume other organisms.
Fungus-like: Saprotrophic slime moulds that feed on decaying matter.
Mixotropes: Organisms that combine multiple feeding strategies.

The range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules

The Rise of Marine Animals

Animal evolution began with simple forms. Sponges are among the most basal animals, lacking nervous, digestive, or circulatory systems. Ctenophores (comb jellies) and cnidarians (such as sea anemones) followed, with cnidarians being the first to organize cells into tissues.

Sponges are perhaps the most basal animals. They have no nervous, digestive or circulatory system.

The evolution of the bilaterian body plan—characterized by a head, tail, and symmetrical sides—allowed for more complex movement and sensory development. This led to a vast array of phyla, including molluscs, arthropods, and echinoderms.

Idealised wormlike bilaterian body plan. With a cylindrical body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

Vertebrates and Tetrapods

Chordates evolved from invertebrate ancestors, eventually leading to fish. This progression moved from jawless fish to cartilaginous fish (like manta rays) and bony fish. Some lobe-finned fish, such as Tiktaalik, developed limb-like fins that enabled the transition to land, paving the way for tetrapods.

Tiktaalik, an extinct lobe-finned fish, developed limb-like fins that could take it onto land.

Ecosystems and Trophic Interactions

Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes

Marine life is organized into complex food webs. At the base are primary producers, mainly cyanobacteria and chloroplastic algae. Diatoms alone account for 50% of the ocean's primary production.

There are over 100,000 species of diatoms which account for 50% of the ocean's primary production.

Plankton and the Pelagic Zone

Plankton are organisms that drift with currents. They are divided into phytoplankton (photosynthetic) and zooplankton (animal-like). Some organisms, known as mixoplankton, can function as both.

Six relatively large variously shaped organisms with dozens of small light-colored dots all against a dark background. Some of the organisms have antennae that are longer than their bodies. — Plankton are drifting or floating organisms that cannot swim against a current, and include organisms from most areas of life: bacteria, archaea, algae, protozoa and animals.

Keystone Species and Productivity

Certain species play a disproportionate role in their environment. For example, sea otters act as a keystone species by controlling sea urchin populations, which protects kelp forests—some of the most productive ecosystems on Earth.

Sea otter, a classic keystone species which controls sea urchin numbers

Biogeochemical Contributions

Sea spray containing marine microorganisms can be swept high into the atmosphere where they become aeroplankton, and can travel the globe before falling back to earth.

Marine life significantly influences the planet's chemistry. Through photosynthesis, marine organisms sequester carbon and release oxygen. Furthermore, the remains of microscopic organisms create biogenic ooze on the ocean floor, which eventually forms sedimentary rock.

Types of Biogenic Ooze
Ooze Type	Mineral Form	Responsible Protist	Skeleton Name
Siliceous	SiO2 (Quartz/Opal)	Diatoms, Radiolarians	Frustule / Skeleton
Calcareous	CaCO3 (Calcite/Aragonite)	Foraminiferans, Coccolithophores	Test / Coccolith

Frequently Asked Questions

Vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas

Lichen on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.

Kimberella, an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).

Taxonomic biodiversity of accepted marine species, according to WoRMS, 18 October 2019.[198][199]

The beroid ctenophore, mouth gaping, preys on other ctenophores.

Crawling motility and food uptake by T. adhaerens

Cnidarians, like this starlet sea anemone, are the simplest animals to organise cells into tissue. Yet they have the same genes that form the vertebrate (including human) head.

Ikaria wariootia, an early bilaterian[244]

Many marine worms are related only distantly, so they form a number of different phyla. The worm shown is an arrow worm, found worldwide as a predatory component of plankton.

First known air-breathing animal to colonise land, the millipede Pneumodesmus newmani,[273] lived in the Early Devonian.[274]

Adult echinoderms have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.

The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.[296]

In chordates, the four above labelled common features appear at some point during development.[291]

The Tully monster, a strange looking extinct animal with eyes like a hammerhead protruding from its back, may be an early jawless fish.

Guiyu oneiros, the earliest-known bony fish lived during the Late Silurian 419 million years ago.

Lobe fins are bedded into the body by bony stalks. They evolved into the legs of the first tetrapod land vertebrates.

Ray fins have spines (rays) which can be erected to stiffen the fin for better control of swimming performance.

Teleosts have homocercal tails, where the upper half mirrors the lower half.

Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Kelp forests are among the most productive ecosystems on the planet.

Marine food web: Mixoplankton paradigm[409]

The drainage basins of the principal oceans and seas of the world are marked by continental divides. The grey areas are endorheic basins that do not drain to the ocean.

Global cumulative human impact on the ocean[422]

Apparent marine fossil diversity during the Phanerozoic[424]

How much of Earth's living space is provided by the ocean?

Oceans provide approximately 90% of the total living space on Earth by volume.

What is the difference between phytoplankton and zooplankton?

Phytoplankton are primary producers that use photosynthesis to create energy, while zooplankton are heterotrophic organisms that consume other plankton or organic matter.

What was the Cambrian Explosion?

The Cambrian Explosion was a major evolutionary radiation event in the early Phanerozoic eon where actively moving eumetazoans (animals) became prevalent in the oceans.

What are keystone species in a marine context?

Keystone species are organisms that have a disproportionately large effect on their natural environment relative to their abundance, such as sea otters controlling urchin populations to maintain kelp forests.

How do marine organisms contribute to land formation?

Certain organisms, most notably reef-building corals, create massive calcium carbonate structures that accumulate over time to form reefs and, in some cases, new land.